So this reaction needs 678 kJ of energy to make it happen, this energy I understand can come from the sun so if you put hydrogen & chlorine in the sun they'll react and produce Hydrogen chloride. OK I'm happy with that, and some reactions only happen if you heat them with a bunsen burner or something, so that's ok too, but what about all the reactions that happen with no energy (apparently) being put in? Eg CaCO3 & HCl, you don't need any heat, you just pour a bit of acid onto chalk and it reacts, so where's the energy coming from to break the bonds? Or am I misunderstanding it all?

The energy comes from the thermal energy in the air. At room temperature, the CaCO3 and HCl molecules have sufficient kinetic energy to overcome the relatively low activation energy required for the reaction to take place. However, if you lower the temperature significantly, the reaction will proceed very slowly because very few of the molecules will have sufficient kinetic energy to react.

Hm, there is so much I don't understand about all this energy stuff that I'm not even sure what question to ask. Can someone suggest a good website where I can read about this? Also how do you know how many Kj are needed to break a bond, are there tables somewhere or do you work it out?